A stress may change the carrier mobility of a channel region in a transistor, thus controlling the stress has become a more and more common method to improve performance of transistors. The stress memorization technology (SMT) and the embedded silicon germanium (embedded SiGe) technology are two common methods to generate stress in a channel region of a transistor, and to increase the carrier mobility of the channel region. If the SMT and/or the embedded SiGe technology are used, when the channel region is on, a trajectory change of a portion of the carriers in the channel region may be prevented. The trajectory change may be caused by a reflection of crystal lattices during a motion of the carriers. Thus, the carrier mobility of the channel region may be improved, and the performance of the transistor may be enhanced.
However, with the continuously shrinking of the critical dimension of semiconductor devices, the size of a channel correspondingly decreases, and a depth of the channel region also decreases. As shown in FIG. 1, when a channel region becomes shallower, a moving direction of a portion of moving carriers in the channel region under the gate 10 may be deflected. The portion of the moving carriers may reach an interface between a dielectric layer 11 and the channel region, and a reflection of the portion of the moving carriers may happen at the interface. Then, a direction and a velocity of the portion of the moving carriers may change. Thus, the mobility of the carriers in the channel region may be reduced, and the performance of the transistor may be reduced as well. The disclosed device structures, methods and systems are directed to solve one or more problems set forth above and other problems.